The present invention relates to a NAND-type non-volatile semiconductor memory device and a method of manufacturing the same.
A NAND-type non-volatile semiconductor memory device (e.g., NAND-type flash memory) includes a word line having an end portion at which a plug is formed for a contact with wiring of an upper layer. The word line is also provided with a fringe in consideration of the misalignment during the formation of the contact plug.
Lithography technology is used for microfabrication in a high degree of integration of elements, by means of illumination method such as off axis illumination (oblique illumination). In the method, a luminous flux for exposure is incident in an oblique direction to a reticle (photomask) by placing an aperture at a position offset from the center of optical axis. The method is also called annular illumination or quasar illumination depending on the shape of aperture, and has an advantage of improved resolution and focal depth.
Dipole illumination technique provides high resolution in one direction. However, at the end of the word line having the fringe, the fringe breaks up the regularity of the structure, which leads to degraded resolution and insufficient margin for exposure. This may causes the fringe to be brought in contact with the adjacent word line, resulting in short-circuit between the word lines.
It is difficult to form fine fringes, and also an addition of another lithography step for forming a fringe increases manufacturing cost. Therefore, it is desired to form a contact plug with the end of a word line being formed in a straight line shape without a fringe.
In forming a contact plug at an end of a word line, it is advantageous to process the contact plug and a contact hole which is formed in a substrate in a peripheral circuit at the same time, in terms of manufacturing cost. The contact hole of the peripheral circuit has a depth greater than that of a memory region.
So, if a contact hole of a memory region is formed under the etching condition for making a contact hole of a peripheral circuit, the former contact hole is excessively etched, and reaches the substrate. A plug provided in the contact hole causes a problem: that is, the plug conducts electricity to the substrate, which results in a short-circuit fault.